(1) Field of the Invention
This invention relates generally to electronic charge pump driving and is specifically applied to a slew rate driver to avoid excessive in-rush current between a dc-dc converter and a load circuit during a turn-on process.
(2) Description of the Prior Art
During the development of power management solutions for mobile electronic devices, particular attention is required to reduce the battery consumption due to leakage of load circuits such as e.g. the CPUs. A common solution is the use of switches between a dc-dc converter and the load circuit (e.g. a processor). These switches isolate electrically the dc-dc converter from the load circuit and require:                a slew rate controlled driver to avoid excessive in-rush current between the dc-dc converter and the load circuit during the turn-on process.        a converter, such as a charge pump, able to supply the driver to a voltage above the battery level.        
FIG. 1 prior art shows a power gating system architecture for such a solution comprising a DC-to-DC converter 1, a charge pump and a slew rate (SR) controlled driver 2, a transistor switch 3, an arrangement of capacitors C1-C3, and a load circuit (CPU) 4. Although this solution is very effective and allows multiple drivers to share the same charge pump, the continuous need of reducing the turn-on time of the switches requires a dramatic increase in the current delivered by the charge pump. A possible solution would be to increase either the size of the charge pump itself or the size of the external capacitor C3.
FIG. 2 prior art illustrates an alternative solution to limit the loading on the charge pump comprising of two drivers drv1 and drv2: drv1 supplied from the battery voltage Vbat and drv2 supplied from the charge pump 20 voltage Vpump. Drv1 is used to charge the switch gate 21 up to Vbat while drv2 is used only to raise the gate voltage from Vbat to Vpump. The in-rush current is controlled via capacitive feedback 22 with an internal capacitor to each driver. In summary solution shown in FIG. 2 prior art comprises one charge pump 20, two drivers drv1 and drv2, and one switch 21.
This approach requires further circuitry and complications to control the transition between voltage domains and to avoid glitches on the gate during the transition.
Furthermore when driver drv2 is active, the voltage drop occurring at the output of charge pump 20 translates into an increase of the resistance of the switch and so to a temporary increase of the power losses.
Although the solution is considered attractive in applications where is required to control multiple switches, it is very inefficient in terms of silicon area (two feedback capacitors and two drivers for each switch) therefore appears to be largely over dimensioned (in both on-chip and off-chip utilization) compared to solutions wherein only one switch is used.
It is a challenge for engineers to provide a method and circuit to avoid excessive in-rush current that do not have the disadvantages of the existing solutions.
There are known patents or patent publications dealing with limiting in-rush currents.
U.S. Patent Application (US 2010/0013548 to Barrow) discloses a charge pump, which uses a current limit resistor to limit in-rush current and peak currents. An additional advantage of such a charge pump is that, when being coupled to a boost converter or other switching converter utilizing an inductive energy storage element, it may avoid unnecessary power dissipation caused by the current limit resistor.
U.S. patent (U.S. Pat. No. 7,880,530 to Ishiyama) discloses a power supply circuit which boosts a given voltage to generate one or more power supply voltages including a charge-pump control circuit including switching elements for generating a boost voltage by a charge-pump operation using charge stored in a flying capacitor, a soft-start circuit which prevents a rush current toward the flying capacitor, and a power supply generation circuit which is connected with a stabilization capacitor and generates a power supply voltage using the boost voltage as a power supply. After the power supply generation circuit has been turned ON in a state in which the charge-pump control circuit generates the boost voltage by the charge-pump operation, the switching elements are turned OFF, and the soft-start circuit generates the boost voltage by a charge-pump operation.
U.S. Patent Application Publication (US 2006/0193156 to Kaishita et al.) proposes a charge pump DC/DC converter circuit including: a monitor circuit that detects a potential difference between terminals of a semiconductor switch that turns on during a first period, so as to output a determining signal corresponding to the potential difference; and each of drive circuits that outputs a drive signal to a semiconductor switch that turns on during a first period, in response to the determining signal. The drive signal increases the on-resistance of the semiconductor switch in proportion to the detected potential difference.
U.S. Patent Application Publication (US 2009/0309633 to Kotowski et al.) proposes a ramp-up circuit for switched capacitor circuits with negative feedback to control the slew rate of in-rush current and other embodiments.
U.S. patent (U.S. Pat. No. 6,738,272 to Yamanaka et al.) discloses a charge pump circuit, wherein a constant current circuit is disposed between an input power supply and an output capacitor. When a power supply is started to turn on, the operation of the charge pump circuit is stopped, and the output capacitor is charged up to a given voltage by the constant current circuit, and thereafter the normal operation of the charge pump is started to limit the rush current. When the power supply is started, the operation is conducted by an oscillator circuit having a small duty ratio, and thereafter the control is replaced by the PFM control having the normal duty ratio, to thereby reduce the rush current as compared with that of the conventional PFM control. When the power supply is started, a pre-driver including a current limiting element is used to drive a driver, resulting in such an advantage that the rush current is reduced as compared with that driven by the conventional pre-driver.
U.S. patent (U.S. Pat. No. 6,744,224 to Ishii) teaches an input voltage detecting circuit for detecting an input voltage provided inside a PFM control charge pump circuit, and potential differences between potentials appearing at gate terminals and potentials appearing at source terminals are reduced by gate voltage controlling circuits for in response to a signal from the input voltage detecting circuit, suppressing gate voltages of switch transistors of a charge pump to suppress a rush current value to thereby reduce a current to prevent generation of a noise.